<select name="num" onchange="getPrice()" id="num" >
       <option value="0" selected="selected">请选择</option>
       <option value="1">1</option>
       <option value="2">2</option>
       <option value="3">3</option>

<input type="text" name="a"  id="b"/>

  <script type="text/javascript">
  function getPrice(){
     var nu = document.getElementById( "num ").value;//得到列表改变后的值
         var c= nu *2;
     document.getElementById("b").value=c;
  
  }
  </script>